R. J. Bouwens, G. D. Illingworth, P. A. Oesch, M. Trenti, I. Labbe, M. Franx, M. Stiavelli, C. M. Carollo, P. van Dokkum, D. Magee
The HUDF09 data are the deepest near-IR observations ever, reaching to 29.5
mag. Luminosity functions (LF) from these new HUDF09 data for 132 z~7 and z~8
galaxies are combined with new LFs for z~5-6 galaxies and the earlier $z\sim4$
LF to reach to very faint limits (<0.05 L*(z=3)). The faint-end slopes alpha
are steep: -1.79+/-0.12 (z~5), -1.73+/-0.20 (z~6), -2.01+/-0.21 (z~7), and
-1.91+/-0.32 (z~8). Slopes alpha ~ -2 lead to formally divergent UV fluxes,
though galaxies are not expected to form below ~ -10 AB mag. These results have
important implications for reionization. The weighted mean slope at z~6-8 is
-1.87+/-0.13. For such steep slopes, and a faint-end limit of -10 AB mag,
galaxies provide a very large UV ionizing photon flux. Adopting typical
parameters, extrapolating the current LF evolution to z>8, and taking alpha to
be -1.87+/-0.13 (the mean value at z~6-8), we derive Thomson optical depths of
0.061. However, this result will change if the faint-end slope alpha is not
constant with redshift. We test this hypothesis and find a weak, but still very
uncertain, trend to steeper slopes at earlier times (dalpha/dz ~ -0.05+/-0.04),
that would increase the Thomson optical depths to 0.079, in excellent agreement
with recent WMAP estimates (tau=0.088+/-0.015). It may thus not be necessary to
resort to extreme assumptions about the escape fraction or clumping factor.
Nevertheless, the uncertainties are large. We show that deeper WFC3/IR+ACS
observations can substantially lessen the uncertainties in the z~5-8 slopes and
further constrain this key parameter in determining the ionizing flux from
galaxies.
View original:
http://arxiv.org/abs/1105.2038
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